Newton's Third Law of Motion

Newton's Third Law of Motion

Middle School

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Learning Objectives

Key Vocabulary

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Force

A push or pull on an object that can cause it to accelerate, change direction, or deform.

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Inertia

An object's resistance to any change in its state of motion, which is directly related to its mass.

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Balanced Forces

When forces on an object cancel out, resulting in zero net force and no change in motion.

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Unbalanced Forces

When the net force on an object is not zero, causing a change in the object's motion.

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Mass

The measure of the amount of matter in an object, which remains constant regardless of location or gravity.

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Weight

The measure of the gravitational force exerted on an object's mass, which can vary depending on location.

Key Vocabulary

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Acceleration

The rate at which an object’s velocity changes, meaning it can be speeding up or slowing down.

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Work

The energy transferred when a force is applied to an object over a specific distance.

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Power

The rate at which work is done or energy is transferred, measuring how quickly work is completed.

Force and States of Motion

• An object’s state of motion can be constant or changing.

• Balanced forces are equal and opposite, causing no change in motion.

• Unbalanced forces are unequal, causing a change in an object’s motion.

Multiple Choice

What is required to cause a change in an object's state of motion?

• An unbalanced force is applied to the object.

• The object has a constant speed.

• Balanced forces are applied to the object.

• The object has a large mass.

Multiple Choice

Which of the following scenarios describes an object experiencing balanced forces?

• A car speeding up from a stop sign.

• A book sitting motionless on a table.

• A ball slowing down as it rolls across grass.

• A rocket accelerating during launch.

Multiple Choice

In a game of tug-of-war, two teams are pulling on a rope with all their strength, but the center of the rope does not move. What is the best explanation for this situation?

• The forces are balanced because they are equal and opposite.

• The forces are unbalanced because one team is pulling harder.

• There are no forces acting on the rope.

• The rope has too much mass to move.

Newton's First Law: The Law of Inertia

• An object at rest will stay at rest unless a force is applied.

• An object in motion continues with the same speed and direction.

• The more mass an object has, the more inertia it has.

Multiple Choice

Which statement best describes Newton's First Law of Inertia?

• An object's state of motion will not change unless an outside force acts on it.

• The heavier an object is, the faster it will fall.

• Every action has an equal and opposite reaction.

• An object will always slow down and stop on its own.

Multiple Choice

What is the relationship between an object's mass and its inertia?

• The object with more mass will have more inertia.

• The object with less mass will have more inertia.

• An object's mass has no effect on its inertia.

• An object's inertia depends only on its speed.

Multiple Choice

A moving train takes a very long time to come to a stop, while a person on a bicycle can stop very quickly. Which statement provides the best explanation for this difference?

• The train has more mass and therefore more inertia, making it harder to stop.

• The bicycle has less mass, so it has a stronger reaction to the force of the brakes.

• The train is moving faster than the bicycle, so it has more energy.

• The bicycle's tires create more friction than the train's wheels.

Newton's Second Law: F = ma

Multiple Choice

What does Newton's Second Law of Motion $$F=ma$$ describe?

• The relationship between net force, mass, and acceleration.

• The concept that an object in motion stays in motion.

• The idea that for every action, there is an equal reaction.

• The relationship between an object's weight and its volume.

Multiple Choice

If you push an object with more net force while its mass stays the same, what happens to its motion?

• The object's acceleration will increase.

• The object's acceleration will decrease.

• The object's acceleration will not change.

• The object will begin to slow down.

Multiple Choice

If the same amount of net force is used to push an empty shopping cart and a full shopping cart, which one will have a greater acceleration and why?

• The empty cart, because it has less mass.

• The full cart, because it is heavier.

• Both carts will accelerate at the same rate.

• Neither cart will accelerate.

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Mass vs. Weight

• Mass is the total amount of matter an object contains.

• An object's mass is always constant and is measured in kilograms (kg).

• Weight is the force of gravity that acts on an object's mass.

• Weight can change with location and is measured in Newtons (N).

Multiple Choice

Which statement best defines an object's mass?

• The total amount of matter that makes up an object.

• The pull of gravity on an object at a specific location.

• The force an object exerts when it is moving.

• The space that an object occupies.

Multiple Choice

What is the key difference between an object's mass and its weight?

• Weight is determined by gravity, which varies by location, while mass is constant.

• Mass is measured in Newtons (N), while weight is measured in kilograms (kg).

• An object always has more weight than it has mass.

• Mass applies to solid objects, while weight applies to liquids and gases.

Multiple Choice

An astronaut brings a rock from the Moon, which has very low gravity, to Earth, which has stronger gravity. What will happen to the rock's mass and weight?

• The rock's mass will stay the same, but its weight will increase.

• The rock's weight will stay the same, but its mass will increase.

• Both the rock's mass and its weight will stay the same.

• Both the rock's mass and its weight will increase.

Newton's Third Law: Action-Reaction

• For every action, there is an equal and opposite reaction.

• Forces always act in pairs between two different objects.

• When you walk, you push the ground, which pushes you forward.

Multiple Choice

Which statement best describes Newton's Third Law of Motion?

• For every action, there is an equal and opposite reaction.

• An object in motion stays in motion.

• Force equals mass times acceleration.

• Forces always act on a single object.

Multiple Choice

What is a key characteristic of action-reaction force pairs?

• They act on two different objects.

• They are always equal to zero.

• They act on the same object.

• One force is always stronger than the other.

Multiple Choice

When a person walks, they push backward on the ground. What is the reaction force that causes the person to move forward?

• The ground pushes the person forward.

• The person's feet pull the ground backward.

• Gravity pulls the person down.

• The person's muscles contract to move their legs.

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Common Misconceptions About Forces

Misconception	Correction
Events like an apple falling 'just happen'.	Any change in motion is caused by a force like gravity.
Mass and weight are the same thing.	Mass is the amount of matter, while weight is the force of gravity.
An object in motion naturally slows down and stops.	Objects slow down due to unbalanced forces like friction and air resistance.
Forces can exist alone.	Forces always occur in equal and opposite action-reaction pairs.

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Summary

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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